Pneumatic stirling cycle cooler with non-contaminating compressor

ABSTRACT

A solenoid drive linear motor actuates bellows which creates pressure pulses traveling in a connecting tube to a cylinder in which a free displacer is located. A pneumatic volume at one end of the cylinder is raised to an average cycle pressure. The pressure pulses propagate through the body of the free displacer to the cold volume end which is to be placed in contact with an element to be cooled and cause the free displacer to be reciprocated in the cylinder resulting in net cooling at the cold volume end. Because the working volume is sealed and the volume changes occur by forces applied to the bellows by the linear drive motor, no contaminants can enter thereby making the instant device a non-contaminating system. The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

United States Horn atent 1 Oct. 16, 1973 [75] Inventor: Stuart B. Horn, Fairfax, Va.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: Aug. 9, 1972 [21] Appl. No.: 279,145

[52] U.S. Cl. 62/6 [51] Int. Cl. F25b 9/00 [58] Field of Search 62/6 [56] References Cited UNITED STATES PATENTS 3,188,821 6/1965 Chellis 62/6 3,220,201 11/1965 Heuchling 62/6 3,321,926 5/1967 Chellis 62/6 3,552,120 l/l97l Beale 62/6 FOREIGN PATENTS OR APPLICATIONS 185,940 0/0000 U.S.S.R 62/6 Primary Examiner-William J. Wye Attorney-Edward J. Kelly [5 7] ABSTRACT A solenoid drive linear motor actuates bellows which creates pressure pulses traveling in a connecting tube to a cylinder in which a free displacer is located. A pneumatic volume at one end of the cylinder is raised to an average cycle pressure. The pressure pulses propagate through the body of the free displacer to the cold volume end which is to be placed in contact with an element to be cooled and cause the free displacer to be reciprocated in the cylinder resulting in net cooling at the cold volume end. Because the working volume is sealed and the volume changes occur by forces applied to the bellows by the linear drive motor, no contaminants can enter thereby making the instant device a non-contaminating system.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

6 Claims, 2 Drawing Figures Elllll PATENTEDUCT 16-1975 3.765.187

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'1 PNEUMATIC STIRLING'CYC'LE COOLER WITH NON-CONTAMINATING COMPRESSOR BACKGROUND OF THE INVENTION In recent years there has been considerable development of cryogenic coolers for infrared detectors. These coolers have suffered from short maintenance'intervals, high acoustic noise, vibrations, large power drain and high production cost. Of importance in this field, is the development of a cold head of low mass that could facilitate rapid scan and surveillance of an area and be utilized, for example, in an airborne system. Building a cryogenic device which is remote from its compressor has heretofore resulted in extremely inefficient systems requiring large-power drains. The following survey discusses several cryogenic devices and the limitations of each:

Stirling cycle this cycle offers the highest cryogenic efficiency of any cooler; but large forces cause high vibrational levels, acoustic noise and short life. A rhombic drive modification, while alleviating most of these problems, would materially increase size, weight, and production costs.

VM coolers the best for balancing acoustic noise, vibrations and long life because of low bearing loads resulting from low required forces. The drawbacks are lower cryogenic efficiency and large production costs.

Solvay and Gifford-McMahon offers remote cooling capability but at the cost of low cryogenic efficiency and large production cost.

The instant invention provides a non-contaminating cryogenic cooler having high efficiency, low acoustic noise, light weight, remote cooling capability and low cost resulting from reduced complexity of construction.

SUMMARY OF THE INVENTION The disclosed cryogenic device utilizes a noncontaminating linear compressor in conjunction with a remotely located coldhead. The linear compressor takes the form of a solenoid drive linear motor operating against a face of a bellows. The bellows are located in a housing which can bebackfilled with a gas such as helium; thus allowing higher pressures to be used in the working space of the bellows. The bellows working space is connected by a tubing to the remote cooler which comprises a cylinder-like member in which is located a free displacer. The free displacer being hollow and filled with stainless steel mesh also serves as a regenerator. One end of the cylinder-like member forms a pneumatic volume which, in operation, is maintained at an average cycle pressure, while the opposite end functions as the cold volume adjacent the cold head.

BRIEF DESCRIPTION OF THE DRAWING FIG. I shows the bellows and linear drive in a cutaway view.

FIG. 2 shows the cryogenic cooler in cross-section.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning to FIG. 1, the disclosed cryogenic cooler utilizes a compressor having motive means therein (not shown) coupled to bellows 11 at the bellows plate 12 by way of a reciprocating shaft 13 and couplingmeans 14. The motive means could, for example, by a be drive linear motor. A sealed housing 15 around the bellows and motive means is filled with a gas such as helium which allows for greater pressure within the working space of the bellows by reducing the pressure differential to which the bellows would otherwise be subjected. Acoustic noise and vibration are eliminated in this linear motor and bellows combination since zero net force results during operation for a properly designed device. A connecting tube 16 passing through plate 19 connects the bellows volume 17 to the cooling device. A leak-tight feed-through 18 provides power access to the linear motor.

The cooler 20, shown in FIG. 2, comprises a cylindrical body 21 in which a free displacer 22 is slidably positioned. The body of the free displacer, made of glass epoxy material, has a hollow core 23 in which a steel mesh 24 is located. The mesh 24 functions as a regenerator. The displacer, so fabricated, acts as a cold seal and as a seal to a pneumatic volume 25 which is an enlarged section 33 adjacent an end of the cylindrical body 21. The connecting tube 16 is in communication with the cold swept volume 26 at the end of the free displacer 22 opposite that end adjacent the pneumatic volume 25 by means of a passageway 27 and the passageway 28 in the free displacer 22. A sealing ring 29 prevents leakage down the cylinder 22 toward the cold swept volume by way other than through the regenerator region of the free displacer 22. A sealing ring 30 located between the passageway 27 and the pneumatic volume 25, however, does permit a pressure buildup in the pneumatic volume so that the pneumatic volume develops an average cycle pressure. The reason therefore and the function will be understood from a description of the cycle presented below. Guide ring 31 adjacent the cold swept volume 26 permits sliding motion of the free displacer. A cold head 32 contacts the item being cooled when the device is in operation.

DESCRIPTION OF THE CYCLE The cryogenic cycle being described is a modification of the Stirling cryogenic cycle. The function of the compressor is to create pressure pulses by compressing and expanding a mechanical volume contained within the bellows. The pressure waves generated by the cyclical movement of the bellows travels along the connecting tube 16 and then though the displacer-regenerator to the cold swept volume 26. The pneumatic volume 25 which is sealed off from the rest of the system by a seal takes on the average cycle pressure. This results from leakage through the seal between the two volumes; the two volumes being the working gas space and the pneumatic volume. Leakage occurs after many cycles so that a steady-state condition prevails. During one cycle leakage between the two volumes is negligible because cycle times are small (about 60 milliseconds). The effect of the pneumatic volume is to provide an average cycle pressure and therefore a restoring force. Starting from the compressor position that gives the average cycle pressure (with the free displacer at the bottom of its stroke, and realizing the mechanical compressor is putting out a sinusoidal pressure wave) the cycle proceeds as follows:

1. The pressure in the cold volume starts increasing due to the compressor going to the end of its stroke. This creates a net force on the displacer toward the pneumatic volume 25. When the net force on the displacer 22 exceeds the static friction force of the two sealing rings 29, 30, the displacer starts to move toward the pneumatic volume. During this step high pressure gas is doing work in moving the displacer 22 toward the pneumatic space and cooling results. When the displacer 22 reaches the end of its stroke, it dwells at that position.

2. Cycle pressure now starts to decrease and soon the cycle pressure is lower than the pneumatic volume pressure. When the pressure difference between the pneumatic volume and the working space is large enough to overcome static friction, the displacer 22 begins to move toward the cold swept volume 26. During this process, low pressure gas in the cold end space is having work done on it. Heating results in this space.

The result of the cycle is that more work is extracted from the gas at the cold end than work is done on it. This cycle offers the unique advantage of an almost square P-V curve. This results from quick movement of the free displacer and long dwell times.

Cycle efficiency is greatly improved as can be seen from the following summation of cycle loss mechanisms:

a. Compressor P-V work b. Compressor bearing losses c. Compressor seal friction d. Compressor seal blow-by e. motor inefficiency In this invention the linear motor eliminates loss b. The bellows eliminates losses d and c. Losses b, c, and d in conventional Stirling cycle cryogenic coolers are quite substantial. Loss e will remain substantially constant; since linear motor efficiencies now approach AC motor efficiencies. Loss a will be slightly larger for this cycle but the net result will be a substantial increase in thermodynamic efficiency.

A typical cooler according to the instant invention would provide better than watt net cooling at 77 K. The stroke would be no greater than approximately Va inch with the bellows subject to a pressure differential of 40-60 psi.

While only one embodiment of the invention has been disclosed, it is to be understood that many variations, substitutions and alterations may be made while remaining within the spirit and scope of the invention which is limited only by the following claims.

I claim:

1. A cyrogenic cooler device comprising:

a housing having a cooling end and a pneumatic volume end;

a free displacer piston means positioned in said housing isolating said cooling end from said pneumatic volume end and capable of recriprocating motion therein;

compressor means including bellows means communicating with said housing for producing pressure pulses in a working gas in said housing in response to a motive force reciprocating said bellows means and causing said free displacer piston means to reciprocate in response to said pressure pulses, whereby net work is extracted from said working gas resulting in cooling.

2. A cooling device according to claim 1 wherein said free displacer piston means is hollow and open at its end adjacent said cooling end and having passageway means adjacent the pneumatic volume end'for communication with said compressor means.

3. A cooling device according to claim 2 including controlled leakage sealing means between said pneumatic volume and said passageway means whereby an average cycle pressure is created in said pneumatic volume; said leakage being slow in relation to the cycle time of the reciprocating free displacer means.

4. A cooling device according to claim 3 wherein said hollow free displacer means contains stainless steel mesh therein permitting said free displacer means to function as a regenerator.

5. A cooling device according to claim 4 wherein said free-displacer piston means and said bellows means are joined by conduit means and form an isolated volume and contain a working gas therein and wherein said motive force is generated by a solenoid drive linear motor reciprocating the bellows means thereby producing the pressure pulses in said working gas.

6. A cooling device according to claim 5 wherein said bellows means and linear motor are sealed in enclosure means and said enclosure means are filled with a gas thereby reducing the pressure differential to which said bellows means are subjected. 

1. A cyrogenic cooler device comprising: a housing having a cooling end and a pneumatic volume end; a free displacer piston means positioned in said housing isolating said cooling end from said pneumatic volume end and capable of recriprocating motion therein; compressor means including bellows means communicating with said housing for producing pressure pulses in a working gas in said housing in response to a motive force reciprocating said bellows means and causing said free displacer piston means to reciprocate in response to said pressure pulses, whereby net work is extracted from said working gas resulting in cooling.
 2. A cooling device according to claim 1 wherein said free displacer piston means is hollow and open at its end adjacent said cooling end and having passageway means adjacent the pneumatic volume end for communication with said compressor means.
 3. A cooling device according to claim 2 including controlled leakage sealing means between said pneumatic volume and said passageway means whereby an average cycle pressure is created in said pneumatic volume; said leakage being slow in relation to the cycle time of the reciprocating free displacer means.
 4. A cooling device according to claim 3 wherein said hollow free displacer means contains stainless steel mesh therein permitting said free displacer means to function as a regenerator.
 5. A cooling device according to claim 4 wherein said free-displacer piston means and said bellows means are joined by conduit means and form an isolated volume and contain a working gas therein and wherein said motive force is generated by a solenoid drive linear motor reciprocating the bellows means thereby producing the pressure pulses in said working gas.
 6. A cooling device according to claim 5 wherein said bellows means and linear motor aRe sealed in enclosure means and said enclosure means are filled with a gas thereby reducing the pressure differential to which said bellows means are subjected. 